The gallbladder of Necturus, a typical epithelium, will be employed in vitro to study the mechanisms of salt and water transport. Our final goal is to describe quantitatively the active and passive transport properties of the cell membranes and the intercellular (paracellular) pathway. This characterization will allow us to understand the mechanisms involved in transport of electrolytes and water and their regulation. Conventional and ion-selective intracellular microelectrodes will be used to determine the properties of these pathways under a variety of experimental conditions, which include changes in the electrolyte composition of the bathing media, use of inhibitors of metabolism and of transport, and other pharmacological agents. From the results of these experiments the ion transport mechanisms at each site (apical membrane, basolateral membrane, paracellular pathway) will be characterized. The uphill and downhill steps of ion translocation will be identified and the contribution of the different pathways to salt transport will be established. Conventional and ion-selective microelectrode measurements in the mucosal and serosal unstirried layers, and in the lateral intercellular spaces will be attempted in order to estimate the osmotic water permeability of the epithelium as a whole and of the two transepithelial pathways. In addition, ionic concentrations in the lateral intercellular spaces and the contribution of the spaces to the paracellular resistances to ion and water flow will be determined. These data should allow a quantitative analysis of the mechanisms of water transport. Analogies in the transport properties of gallbladders of widely different species of vertebrates, and similarities between the gallbladder and other epithelia allow us to think that the results of this research will be applicable to leaky epithelia as a whole.